Dual-cell high speed downlink packet access (HSDPA) has been introduced in the third generation partnership project (3GPP) Release 8 as part of the continuing evolution of high speed packet access (HSPA) systems. This feature allows simultaneous downlink (DL) transmission and reception on two adjacent carriers on the high-speed channels. As part of 3GPP Release 9, the feature was extended to support DL transmission and reception on non-adjacent DL carriers, (e.g., carriers in different frequency bands). The ability to support simultaneous reception on non-adjacent carriers significantly impacts the radio frequency (RF) design of a wireless transmit/receive unit (WTRU) including separate RF receivers.
In order to support inter-frequency and inter-radio access technology (RAT) handovers, a WTRU performs measurements on other frequencies and/or other RATs and report the measurements to the radio access network. In case where a WTRU is equipped with a single RF receiver, the WTRU performs the inter-frequency and/or inter-RAT measurements during the measurement gaps. During the measurement gaps, a downlink transmission to the WTRU is interrupted, and the WTRU is allowed to tune its RF receiver to other frequencies and/or RATs to perform the inter-frequency and/or inter-RAT measurements. In 3GPP universal mobile telecommunication systems (UMTS) wireless communications systems, these measurements gaps are referred to as compressed mode (CM) gaps. In accordance with the current 3GPP UMTS specification, both the DL reception and the UL transmission are interrupted during the CM gaps, which causes a degradation of service.